Synthetic or artificial turf is used more and more to replace natural grass turf on playing surfaces, in particular on sport fields like fields for playing football, rugby, tennis, golf, hockey, baseball etc. In order to provide a somewhat resilient surface, a top-dressing can be applied onto the backing layer. The thickness of this top-dressing is smaller than the height of the tufts so that the grass-like filaments project above the top-dressing. A top-dressed synthetic turf is disclosed for example in U.S. Pat. No. 4,337,283.
In practice, the top-dressing of so-called third generation synthetic grass fields usually consists of a hard layer and on top a layer of resilient granules, as for instance disclosed in WO 01/98589. This document discloses a synthetic grass assembly for installation on a supporting substrate comprising a pile fabric with a flexible sheet backing and a plurality of upstanding synthetic ribbons of a selected length. An infill layer of particulate material, selected from the group consisting of hard and resilient granules, is disposed interstitially between the upstanding ribbons upon the upper surface of the backing with a depth less than the length of the ribbons. The infill layer in particular comprises a bottom course of hard granules, disposed upon the top surface of the backing and a top course substantially exclusively of resilient granules disposed upon the bottom course.
Frequently applied resilient granular materials that may be used as infill materials may include mixtures of granulated rubber particles like SBR (styrene butadiene rubber) recycled from car tires, EPDM (ethylene-propylene-diene monomer), other vulcanised rubbers or rubber recycled form belts. However, these rubbers have several important disadvantages. In particular, they can not be re-used and have a limited life span since they loose there properties throughout use. Furthermore, the use of this type of rubbers in a top-dressing layer of a synthetic turf does not provide sufficient resilience or shock absorption. Most of the synthetic turf for football fields consists nowadays of pile fabric made of fibrillated yarn. This fibrillated yarn is usually made of polyethylene or of a mixture of polyethylene and polypropylene and is tufted on a machine with a needle distance of between ⅝″ (≈15.8 mm) and 3/16″ (≈4.7 mm). A drawback of the used fibrillated yarns is that they have a relatively low wear resistance and that a post-fibrillation with a rigid (steel) brush is required after having laid the synthetic turf. The post-fibrillation is required to separate the different filaments of the fibrillated yarn from one another in order to hide the topdressing better from view and in order to achieve the look of natural grass. A drawback of such a post-fibrillation is however that the pile yarn is damaged. In addition to synthetic turf made of fibrillated yarn, there is also synthetic turf made of so-called monotape or monofilament yarn. The difference between monotape and monofilament yarn is that, for the production of monotape yarn, a film is extruded which is cut into small bands whilst for the production of monofilament yarn the bands forming the monofilaments are separately extruded. A drawback of synthetic turf made of monotape or monofilaments is that the top-dressing is less stabilised against shifting and/or erosion and that the rubber granules are less hampered from jumping up. In practice, most of the monotape or monofilament yarns used to make artificial turf are moreover made of polypropylene which offers better resilience properties than polyethylene but which has a higher coefficient of friction so that burning wounds occur much quicker, for example when falling or making a sliding on the synthetic turf surface. Synthetic turfs formed by yarns made of polyamide have been reported in the art, and are for instance disclosed in U.S. Pat. No. 3,940,522 and WO 99/04074. In U.S. Pat. No. 3,940,522 synthetic turfs are described comprising grass-like synthetic fibers and crimped fibers. One or more of the grass-like fibers are combined with an appropriate multi-fiber strand of crimped and/or latently crimpable fibers. The fibers are made of polyamides such as nylon 6, nylon 6,6, nylon 6,10, nylon 6,12, and copolymers and blends of these. WO 99/04074 discloses yarns containing polyamide in combination with a polyolefin compound for producing artificial grass. A major disadvantage of this type of synthetic turfs formed by yarns made of polyamide is that the turfs show high sliding resistance, and a high coefficient of friction so that burning wounds occur much quicker, for example when falling or making a sliding on the synthetic turf surface.
In practice there is also a synthetic turf on the market comprising alternating rows of tufts made of fibrillated yarn and rows of tufts made of monofilament yarns. An advantage of such a combination is that the turf has an appearance which immediately resembles more natural grass. However, a post-fibrillation or several months of wear is still required to make the synthetic turf look like natural grass. A further drawback of this combination is that, due to the fact that the fibrillated yarn wears more quickly than the monofilament yarns, the difference in wear pattern between the fibrillated yarn and the monofilament yarns can clearly be seen after a more prolonged time of use.
It is an object of the present invention to provide a new type of synthetic turf that has more the look of natural grass, but which solves the problem of the difference in wear pattern which can be seen after a prolonged time of use of the prior art synthetic turf. It is another object of the invention to provide a synthetic turf having improved resilience, shock absorption and safeness (e.g. reducing the risks for skin burns when making a sliding on the turf surface).